In his influential book, A Secular Age, philosopher Charles Taylor contrasts the "porous self" of the Middle Ages with the "buffered self" of modernity. Aquinas can be seen as a figure who stands between these two worlds.

Taylor argues that our medieval ancestors inhabited an enchanted universe that was alive with magical powers. The boundary between the individual human and the rest of nature was porous, and humankind belonged within a vast organic cosmos of seen and unseen beings. This helps us to understand why Aquinas regards angels and demons as real creatures who invite philosophical reflection as much as any other life form.

Modern rationalism sealed off the mind from such ideas, eradicating the hidden forces of nature and subduing the power of the imagination. But it was Aquinas and his scholastic contemporaries who set in motion this transformation in the order of western knowledge, when they married philosophy to theology. Aquinas might have been appalled to know that eventually the scaffolding of reason would no longer rest on the bedrock of faith, but he played a significant role in erecting that scaffolding. That is why, despite his medieval context, we might acknowledge him as a father of modernity.

Aquinas's understanding of the human soul was very different from our modern concept of the mind. It was perhaps closer to the Freudian idea of the soul. Freud was an atheist, but in German his work refers to the soul (seele), wrongly translated into English as "mind".

The mind, or soul, that psychoanalysis reveals tells us that our unconscious is home to the fantasies, desires and terrors of a more primordial self, so that a turbulent otherness seethes within the ostensibly rational and autonomous mind of scientific modernity. We need only think of computer-generated creatures to realise that angels, demons, gargoyles and monsters retain their ability to fascinate and terrify us. So maybe the differences between Aquinas's world and our own are not that great, if we expand what we mean by "the mind".

Nature, including human nature, is for Aquinas essentially good because existence itself is good. He said repeatedly that "grace perfects nature" – our desire for God makes us more, not less, natural. A more pessimistic view emerged when the theologians of the reformation set grace over and against nature, insisting that the goodness of human nature had been entirely destroyed by original sin. This paved the way for the Enlightenment and the scientific revolution when nature, including human nature, was finally subjugated to the control of the rational mind, severed from its scriptural and theological inheritance.

For Aquinas, reason is natural to our species, because it is what a creature needs if it has choice. Other animals, lacking freedom, naturally do what is good for them according to the kind of species they are. They too have souls, but the human is uniquely endowed with intellect and will because we are made in the image of God and therefore enjoying a relative freedom in relation to the world. Our intellect enables us to imagine the world as other than it is, and to weigh up the choices which present themselves to reason through our senses and experiences.

Intellect, will, and reason, all have precise, technical meanings for Aquinas.

Reason is the natural activity by way of which we organise and conceptualise these experiences, but the intellect enables us to discern the truth within them. It is the intellect that transforms knowledge into wisdom as we are drawn to the goodness and beauty of God by the diverse goods of the world, and enables us to choose those goods that best express our desire for the ultimate good.

Our will directs our activities so that we can act on these insights. This has an important consequence. For Aquinas the will is about love and desire, not about power and control. The modern concept of willpower emerged when "man" set reason against nature and began to use his will as an expression of control, rather than of desire in relation to the things of the world.

But our desire is influenced by sin, so that we are vulnerable to addictive cravings which destroy our freedom and our capacity for happiness. The good life therefore requires us to discipline desire if we are to be truly free and happy. But it's important to realise that Aquinas does not think we can be motivated by evil. We can be mistaken – innocently or even culpably – in what we think would be good for us, but we cannot be motivated by evil, because evil cannot be more than an absence of the good.

Aquinas is first and foremost a reasoned Christian optimist. Life is very good, because all beings participate in God's being. As one scholar writes, "the being of God is the doing of the world". We'll start from there next week.